Nanowire based semiconductor devices such as LEDs (Light Emitting Diodes), FETs (Field Effect Transistors), diodes, solar cells and detectors are comprised of semiconductor nanowires or arrays of semiconductor nanowires grown on the surface of a substrate, for example silicon, sapphire, GaAs, GaP, GaN. Often, a planar buffer layer is grown first on the substrates and subsequently the semiconductor nanowires or the arrays of semiconductor nanowires are grown on the surface of the buffer layer. The buffer layer is used as the base layer for growing the nanowires. Additionally, it can serve for as an electrical current transport layer.
The basic process of nanowire formation on substrates is by particle assisted growth or the so-called VLS (vapour-liquid-solid) mechanism described in U.S. Pat. No. 7,335,908, as well as different types of Chemical Beam Epitaxy and Vapour Phase Epitaxy methods, which are well known. However, the present invention is limited to neither such nanowires nor the VLS process. Other suitable methods for growing nanowires are known in the art and are for example shown in the international application WO 2007/104781. From this it follows that nanowires may be grown without the use of a particle as a catalyst.
Before growth, the surface of the substrate or the buffer layer is lithographically patterned or otherwise prepared to define where to grow the nanowires. It is beneficial to have the whole area of the wafer patterned homogeneously to ensure uniform growth conditions all over the wafer.
A functional semiconductor device typically contains an active area and one or more areas where contacts are provided. For devices that utilize a buffer and/or a substrate as a current transporting means it is necessary to electrically contact this buffer layer or substrate. Prior art solutions require selective nanowire growth only in the active area or subsequent selective removal of the nanowires from the contact area such that a flat surface of the buffer or of the substrate is exposed. However, standard processes are difficult to apply for nanowire removal, since typical etch processes do not work well for this kind of structure and selective nanowire growth yields non-uniform growth conditions. The present invention offers a solution to avoid these limitations.